Synchronous generators operate within electrical systems to provide uninterrupted power to consumers. A variety of methods are used to protect the generator and thereby the electrical system from internal and external faults. Internal faults in the generator, such as stator winding ground faults, are an important concern to generation utilities worldwide.
One known system and method for 100% stator ground fault protection uses generator 3rd harmonic voltages. In this system and method, operating data from an in-use generator and the larger electrical system are collected and analyzed to set fixed settings into a relay in order to provide stator ground fault protection. These fixed relay settings determine when the relay is to trip or issue an alarm. The fixed relay settings must include extra tolerances (margins) to allow for normal 3rd harmonic voltage variations that occur while operating generators and electrical systems. Moreover, this solution cannot detect stator ground faults in an off-line generator prior to being connected to the electrical system because operating data is required to set the fixed relay settings.
Another known system and method utilizes sub-harmonic voltage injection. However, sub-harmonic voltage injection requires additional equipment to be placed into service, which requires field commissioning. And yet the system and method still require fixed relay settings. Furthermore, the total capacitance-to-ground of the generator stator windings, bus work, and delta-connected transformer windings of the unit transformer windings must be known to ensure the relay settings are correctly determined. Due to higher cost and maintenance of the added equipment, this solution is less attractive compared to the first solution. However, in contrast to the first solution, this solution can detect stator ground faults in an off-line generator prior to being connected to the electrical system.
In addition, known 100% stator ground fault protection systems and methods using 3rd harmonic voltages lack security at low power factors and during unusual power system conditions. Security means the degree of certainty that a protective relay will operate correctly for normal operating conditions of protected equipment. This lack of security causes false tripping or false alarms for the relays and leads to shutdown of the generator, which in turn leads to a costly loss of production and costly testing of the generator. Hence, it is desirable to develop a secure, low-cost stator ground fault protection methods, devices, and systems.
The present invention overcomes one or more of these problems.